JP6178102B2 - Compression coil spring and method of manufacturing the same - Google Patents

Description

  The present invention relates to a compression coil spring used, for example, in an automobile engine or clutch, and more particularly to a compression coil spring having excellent fatigue resistance even in a use environment under high stress and a method for manufacturing the same.

  In recent years, demands for reducing fuel consumption for automobiles have become stricter year by year due to environmental problems, and there is a strong demand for smaller and lighter automobile parts than ever. In response to this demand for small size and light weight, for example, compression spring components such as valve springs used in engines and clutch torsion springs used in clutches are made with high strength materials and surface treatment. Research on surface strengthening has been actively conducted, and as a result, improvements in fatigue resistance and sag resistance, which are important characteristics of coil springs, have been attempted.

  Generally, the manufacturing method of a coil spring is divided roughly into a hot forming method and a cold forming method. The hot forming method is a coil spring in which cold forming is difficult due to its poor workability such as a large wire diameter d and a small spring index D / d which is a ratio of the coil average diameter D to the wire diameter d. Carbon steel or spring steel is used as the coil spring wire. In the hot forming method, the wire rod is heated to a high temperature so that it can be easily processed, coiled into a coil spring shape, coiled into a coil spring shape, and after quenching and tempering, shot peening and setting are further performed. Has become fatigue resistance and sag resistance. In the hot forming method, coiling with a coreless metal is technically very difficult, so that it has not been put into practical use so far. Therefore, in the hot forming method, it is indispensable in the prior art to use a cored bar, and as a coil spring that can be molded, the degree of freedom of shape is low compared to the cold forming method that can be coiled with a coreless bar.

  On the other hand, valve coil and clutch torsion spring class compression coil springs can be cold formed because of their relatively small wire diameter. In addition, it is easy to obtain high dimensional accuracy because it does not involve transformation or thermal expansion / contraction due to heating, and mass productivity (tact, cost) due to processing speed and equipment costs is high. Conventionally, a cold forming method has been adopted. In addition, for this cold forming method, a coreless metal forming technique has been established, and the high degree of freedom in the shape of the coil spring is also a major factor in the use of the cold forming method. The manufacturing technology of compression coil springs of the valve springs and clutch torsion springs class by has never existed. In the cold forming method, a hard wire such as a carbon steel wire, a hard steel wire, a piano wire, or a spring steel wire has been conventionally used as the coil spring wire. However, in recent years, there has been a demand for higher strength of materials from the viewpoint of weight reduction, and expensive oil tempered wires have been widely used.

  In the cold forming method, as shown in FIGS. 1D and 1E, the wire is cold coiled into a coil spring shape, and after annealing, shot peening and setting are performed as necessary. Here, the purpose of annealing is to remove the residual stress caused by the processing that hinders the improvement of the fatigue resistance of the coil spring. Together with the application of compressive residual stress to the surface by shot peening, Contributes to improved fatigue resistance. In addition, about the coil spring used by high load stress like a valve spring and a clutch torsion spring, the surface hardening process by a nitriding process is performed as needed before shot peening.

There are many studies aimed at further improving fatigue resistance. For example, Patent Document 1, oil-tempered over line for cold forming has been described a technique for improving the fatigue resistance by utilizing deformation-induced transformation of retained austenite has been disclosed. Patent Document 2 discloses a technique for improving fatigue resistance by applying a large compressive residual stress to a surface of a wire subjected to nitriding treatment by multistage shot peening at different projection speeds.

  In Patent Document 1, residual stress is generated in the coil spring after coiling. This residual stress, particularly the tensile residual stress in the direction of the linear axis generated on the inner surface of the coil, is an impediment to improving fatigue resistance as a coil spring. Normally, annealing is performed to remove the residual stress due to this processing, but even with the wire in Patent Document 1 having a high resistance to temper softening, the residual stress is completely maintained while maintaining the strength of the desired wire. This is difficult to remove and is well known to those skilled in the art. Therefore, when shot peening is performed afterwards, it is difficult to apply sufficient compressive residual stress to the wire surface due to the effect of tensile residual stress remaining on the inner diameter side of the coil due to processing, and sufficient fatigue resistance as a coil spring Can't get. Further, elements such as V and Mo that contribute to the improvement of the temper softening resistance are expensive. Therefore, the wire is very expensive, and naturally the coil spring as a product is also expensive.

  Moreover, in patent document 2, the compression residual stress of the wire spring surface vicinity (henceforth "surface") of a coil spring is about 1400 MPa, As a coil spring used under the high load stress of a valve spring or a clutch torsion spring class, The compressive residual stress is sufficient for suppressing crack initiation on the surface. However, as a result of improving the compressive residual stress on the surface, the compressive residual stress inside the wire is reduced, and the effect of the compressive residual stress is poor on cracks inside the wire starting from inclusions. Become. That is, with the means according to Patent Document 2, the energy given by shot peening is limited, that is, although the change in compressive residual stress distribution is given, it is difficult to greatly improve the sum of compressive residual stresses. It is not considered to eliminate the influence of the residual stress caused by the above-described processing. Therefore, the effect of improving the fatigue resistance is poor with respect to the wire having the same strength.

Japanese Patent No. 3595901 JP 2009-226523 A

  As described above, the conventional manufacturing method and Patent Documents 1 and 2 and the like are difficult to meet the recent demands for further improvement of fatigue resistance under high stress and cost reduction. The In addition, oil tempered wires, which are currently mainstream for cold forming, are expensive. Above all, oil tempered wires to which higher elements such as Ni, V, and Mo are added for performance improvement are very expensive. Furthermore, since the residual stress due to the processing cannot be completely eliminated by the annealing treatment after forming, the performance of the wire cannot be fully utilized.

  Under such a background, the present invention eliminates the tensile residual stress due to the coiling process and forms a C-concentrated layer on the surface of the wire, thereby providing an appropriate compressive residual stress distribution to the formed wire. An object of the present invention is to provide a highly durable compression coil spring using a simple wire and a method for manufacturing the same.

  The inventors of the present invention have conducted extensive research on the fatigue resistance of coil springs. And in order to obtain a big compressive residual stress, it came to the thought that it is important to eliminate the tensile residual stress at the time of coiling, and to obtain the effect of shot peening and setting performed later effectively. Therefore, a method for eliminating the tensile residual stress of the spring wire before the shot peening process was studied. As a result, focusing on the fact that the residual stress can be eliminated by heating the coil spring wire to the austenite region, coiling is performed while the coil spring wire is heated to the austenite region, thereby eliminating the residual stress caused by the processing. It was found that the effects of shot peening and setting performed later can be obtained efficiently.

  In the heating stage up to the austenite region, heating in a shorter time leads to suppression of coarsening or refinement of the prior austenite crystal grain size (hereinafter referred to as “crystal grain size”). The crystal grain size is closely related to the fatigue resistance, and the refinement of the crystal grain size is effective for improving the fatigue resistance. Therefore, by heating the coil spring wire in a short time and performing hot processing, it becomes possible to manufacture a spring with more excellent fatigue resistance in combination with eliminating residual stress caused by processing.

  Furthermore, by carburizing the coil spring and forming a C-enriched layer on the inner surface of the spring inner diameter side, the yield stress is improved with high hardness in the vicinity of the surface on the inner diameter side of the spring where the maximum tensile stress acts during compression. The effect of shot peening performed later can be obtained efficiently. Here, if the carburizing process is performed during hot coiling, the carburizing process can be performed efficiently.

  Moreover, the effect of the shot peening or setting performed later can be efficiently obtained by heating the coil spring wire to the austenite region after the cold coiling to eliminate the residual stress due to the processing. In the case of cold coiling, the carburizing process can be efficiently performed by performing the carburizing process simultaneously with heating after coiling.

  That is, the compression coil spring according to the present invention is a compression coil spring using a steel wire material having a mass% and C of 0.80% or less, and exceeds the average concentration of C contained in the steel wire material at the spring inner diameter side surface layer portion. It has a C-concentrated layer, and the surface C concentration and the C-concentrated layer depth are continuously decreased along the circumference of the transverse section from the inner diameter side of the spring toward the outer diameter side in an arbitrary wire cross section. It is characterized by that.

  Here, C contributes to strength improvement, but if the content of C exceeds 0.80% by weight, the toughness is lowered and cracking is likely to occur. For this reason, content of C shall be 0.80 weight% or less. If the C content is less than 0.45% by weight, the effect of improving the strength cannot be obtained sufficiently, so that the fatigue resistance and sag resistance are insufficient. Therefore, C is preferably 0.45% by weight or more.

  In the present invention, the spring inner diameter side surface layer portion has a C-concentrated layer exceeding the average concentration of C contained in the steel wire, and the surface C concentration and C-concentrated layer depth in the transverse cross section of the arbitrary wire rod It is characterized by decreasing continuously from the spring inner diameter side to the outer diameter side along the surface circumference. When the coil spring is compressed, the maximum tensile stress acts on the inner surface portion of the spring inner diameter side. In the present invention, a C concentrated layer is formed in the portion by carburizing treatment to improve the yield stress. A large surface compressive residual stress can be applied by peening. In addition, the presence of the hard C-concentrated layer can improve the surface roughness after shot pinning, and has the effect of further improving fatigue resistance. Since the surface C concentration and the C-enriched layer depth continuously decrease along the transverse cross-section circumference from the spring inner diameter side to the outer diameter side, the yield stress distribution in the coil spring is reduced. Corresponding to the stress distribution, the C-enriched layer can be efficiently applied to the portion that needs to be strengthened without causing a sudden change in elastic strain on the circumference of the cross section. For this reason, the performance (fatigue resistance) equivalent to the case where the whole coil spring is carburized can be exhibited.

  In order to sufficiently obtain the effect of the C-enriched layer, the maximum C concentration in the C-enriched layer is preferably 0.7 to 0.9%, and the C-enriched layer (carburized depth) is 0 from the wire surface. It is preferable to be formed to a depth of 0.01 to 0.1 mm. When the maximum C concentration of the C concentrated layer is less than 0.7%, or when the thickness of the C concentrated layer is less than 0.01 mm, the effect of carburization cannot be sufficiently obtained. On the other hand, when the maximum C concentration of the C-concentrated layer exceeds 0.9%, C that cannot be dissolved in the matrix phase precipitates as coarse carbides, which becomes a fracture starting point due to the difference in elastic strain between the carbides and the matrix phase. It easily causes a decrease in fatigue resistance. When the thickness of the C-concentrated layer exceeds 0.1 mm, a high-temperature treatment is required on the premise of a short-time treatment. As a result, the crystal grain size becomes coarse and fatigue resistance is reduced. . On the other hand, since the carburizing depth depends on the carburizing time, it takes a long processing time to perform carburizing to a depth exceeding 0.1 mm while maintaining the crystal grain size. Therefore, in the first manufacturing method and the second manufacturing method of the compression coil spring of the present invention, which will be described later, processing for a long time leads to a decrease in the supply speed of the steel wire, and during the coiling and the subsequent large Normal quenching becomes difficult because the temperature decreases. In the third manufacturing method, carburizing treatment for a long time is not practical from the viewpoint of production capacity.

  Next, the manufacturing method of the compression coil spring of this invention is described. In the first method for producing a compression coil spring of the present invention, a coiling step of hot forming a steel wire by a coil spring forming machine, and a quenching step of quenching a coil that has been separated after coiling and is still in the austenite region as it is Then, a tempering step for tempering the coil, a shot peening step for imparting compressive residual stress to the surface of the steel wire, and a setting step are sequentially performed. Here, the coil spring forming machine is continuously supplied from the rear after a feed roller for continuously supplying the steel wire, a coiling portion for forming the steel wire into a coil shape, and coiling the steel wire with a predetermined number of turns. It has the steel wire which comes and the cutting means for cutting. The coiling unit includes a wire guide for guiding the steel wire supplied by the feed roller to an appropriate position of the processing unit, and a coiling for processing the steel wire supplied via the wire guide into a coil shape. A coiling tool including a pin or a coiling roller and a pitch tool for adding a pitch are provided. Further, the coil spring forming machine has a heating means for raising the temperature of the steel wire to the austenite region within 2.5 seconds between the outlet of the feed roller and the coiling tool. Then, during the period from heating to quenching, hydrocarbon gas is directly applied to the surface of the steel wire from the gas blowing nozzle located in the direction of the outer diameter when formed into a spring shape in the radial direction of the steel wire. Carburizing process is performed.

  Moreover, in the 2nd manufacturing method of the compression coil spring of this invention, the carburizing process which forms C concentrating layer in the surface layer part of steel wire, the coiling process which hot-forms steel wire with a coil spring forming machine, and coiling After that, the quenching process in which the coil that has been separated and the temperature is still in the austenite region is quenched as it is, the tempering process in which the coil is tempered, the shot peening process in which compressive residual stress is applied to the surface of the steel wire, and the setting process are sequentially performed. Do. The means for forming the C-enriched layer in the carburizing step is hydrocarbon on the surface of the steel wire heated from the gas spray nozzle located in the direction of the outer diameter when formed into a spring shape in the radial direction of the steel wire. System gas is sprayed directly. The coil spring forming machine used in the coiling process is the same as that used in the first manufacturing method of the present invention. The heating means is high-frequency heating, and the high-frequency heating is performed so as to be concentric with the steel wire on the passage of the steel wire in the wire guide or on the passage of the steel wire in the space between the steel wire outlet end and the coiling tool in the wire guide. A heating coil is arranged. And the carburizing process and the coiling process are continuous processes in which there is no separation of the steel wire material.

  Furthermore, in the third manufacturing method of the compression coil spring of the present invention, a coiling step of forming a steel wire by a coil spring forming machine, a heating and quenching step of heating and quenching the coil to an austenite region within 20 seconds, A tempering step for tempering the coil, a shot peening step for imparting compressive residual stress to the surface of the wire, and a setting step are sequentially performed. The heating means in the heating and quenching process is high-frequency heating, and carburizing in which hydrocarbon gas is directly blown onto the surface of the steel wire rod from the gas blowing nozzle located in the outer diameter direction of the formed coil spring between heating and quenching A process is performed.

  In the first to third manufacturing methods of the compression coil spring of the present invention, the direction (first and second manufacturing methods) which becomes the outer diameter side when formed into a spring shape in the radial direction of the steel wire, or The hydrocarbon-based gas is directly sprayed onto the surface of the steel wire from a gas spray nozzle located in the outer diameter direction (third manufacturing method) of the formed coil spring. When hydrocarbon gas is blown directly from the gas blowing nozzle onto the surface of the steel wire, the surface of the blown steel wire is cooled and the temperature drops, and there is little or no reaction with the hydrocarbon gas. is there. On the other hand, on the inner diameter direction side of the coil spring, the hydrocarbon-based gas goes around the steel wire and contacts the surface, but the surface is not cooled, and the reaction with the hydrocarbon-based gas is actively performed. Accordingly, the surface C concentration and the C enriched layer depth continuously decrease along the transverse cross-section circumference from the spring inner diameter side toward the outer diameter side. Moreover, since the steel wire is heated in a short time and then carburized by directly blowing hydrocarbon gas, the growth of crystal grains can be suppressed, the crystal grains can be made fine, and the fatigue resistance can be improved. Can do. In addition, in order to acquire the above effects reliably, it is desirable that the tip of the gas spray nozzle be separated from the steel wire surface by 0.1 to 10.0 mm.

  Here, it is preferable that the surface temperature of the steel wire at the time of blowing the hydrocarbon-based gas is 850 to 1150 ° C., and the dynamic pressure of the hydrocarbon-based gas on the surface of the wire is 0.1 to 5.0 kPa. . According to this carburizing condition, carburizing can be performed efficiently in a short time while preventing a significant decrease in the crystal grain size of the wire. In the first to third manufacturing methods of the compression coil spring of the present invention, it is preferable that the main component of the hydrocarbon gas is any one of methane, butane, propane, and acetylene.

  In the said manufacturing method, a tempering process is performed in order to temper the coil hardened | cured by the hardening process to the coil which has appropriate hardness and toughness. Therefore, when desired hardness and toughness can be obtained with quenching, the tempering step may be omitted. In the shot peening process, multi-stage shot peening may be performed, and furthermore, low temperature aging treatment for the purpose of recovering the surface elasticity limit may be combined as necessary. Here, the low temperature aging treatment can be performed after the shot peening process or between each stage of the multi-stage shot peening, and shot peening with a shot having a particle diameter of 0.02 to 0.30 mm is performed as the final stage in the multi-stage shot peening. When applied, it is preferable to perform the pretreatment to further increase the compressive residual stress on the outermost surface. Although there are various methods such as cold setting and hot setting as the setting applied to the coil as a settling prevention process in the setting process, it is selected as appropriate according to desired characteristics.

  According to the first and second manufacturing methods of the compression coil spring of the present invention, since hot coiling is performed by the coil spring forming machine, it is possible to prevent generation of residual stress due to processing. And since a steel wire is heated up to an austenite area within 2.5 seconds, the coarsening of a crystal grain can be prevented and the outstanding fatigue resistance can be acquired. Moreover, since the carburizing treatment is performed, the surface of the steel wire can be made high in hardness, and compressive residual stress can be effectively applied by shot peening performed later. Particularly, in the first manufacturing method of the compression coil spring of the present invention, the carburizing process is performed using the heat at the time of hot coiling, and therefore the carburizing process can be performed efficiently.

  Further, according to the third manufacturing method of the compression coil spring of the present invention, since the coil is heated to the austenite region within 20 seconds and quenched, the tensile force generated by cold coiling while preventing coarsening of crystal grains is prevented. Residual stress can be eliminated. Moreover, since the carburizing process is performed using the heat at the time of heating and quenching, the carburizing process can be performed efficiently. From these, compressive residual stress can be effectively applied by shot peening performed later, and excellent fatigue resistance can be obtained.

The present invention is a carbon steel wire, hard steel wire, piano wire, spring steel wire, carbon steel oil temper wire, chrome vanadium steel oil temper wire, silicon chrome steel oil temper wire, silicon chrome vanadium steel oil temper used as a spring. Applicable to lines. In particular, it is preferable to apply to inexpensive carbon steel wires, hard steel wires, piano wires, and spring steel wires. This is because the above-described manufacturing method, to obtain the fatigue resistance of the spring over the prior cold forming spring using expensive oil-tempered over line exclusive elements be utilized is added inexpensive wire This is because it can.

  According to the present invention, a highly durable compression coil spring is obtained using an inexpensive wire by eliminating tensile residual stress due to coiling and imparting an appropriate compressive residual stress distribution to the formed wire. be able to.

It is a figure which shows an example of the manufacturing process of a coil spring. It is the schematic of the shaping | molding part of the coiling machine in embodiment of this invention. It is the schematic which shows the high frequency heating coil installation position in 1st Embodiment of this invention. It is the schematic which shows the high frequency heating coil installation position in 2nd Embodiment of this invention. It is the schematic which shows the example of a change of the high frequency heating coil installation position in 1st Embodiment of this invention. It is the schematic which shows the high frequency heating coil installation position in 3rd Embodiment of this invention. (A) is sectional drawing which shows the compression coil spring which investigated C concentration layer in the Example of this invention, (B) is a figure which shows C concentration layer in Invention Example 8, (C) is C in Invention Example 15. It is a figure which shows a concentrated layer. It is a graph which shows the relationship between the C density | concentration measurement position and surface C density | concentration in invention example 8 and invention example 15.

  Hereinafter, embodiments of the present invention will be specifically described. In the present invention, it is desirable to have the following components. In the following description, “%” means “% by weight”.

Si: 0.15 to 2.50%
Si is effective for deoxidation of steel and contributes to improvement of strength and resistance to temper softening. If the Si content is less than 0.15%, these effects cannot be obtained sufficiently. On the other hand, if the Si content exceeds 2.50%, the toughness is lowered and cracks are likely to occur, and decarburization is promoted to cause a reduction in the wire surface strength. For this reason, it is desirable to contain 0.15 to 2.50% of Si.

Mn: 0.3 to 1.0%
Mn contributes to improvement of hardenability. When the Mn content is less than 0.3%, it becomes difficult to ensure sufficient hardenability, and the effect of S fixing (MnS generation) that is harmful to ductility becomes poor. On the other hand, when the content of Mn exceeds 1.0%, ductility is lowered, and cracks and surface scratches are likely to occur. For this reason, it is desirable to contain 0.3 to 1.0% of Mn.

  Other elements may be further added. That is, in the present invention, one or more elements of Cr, B, Ni, Ti, Cu, Nb, V, Mo, W, etc., which are generally used as the component composition of spring steel, are set to 0. 0.005 to 4.5%, which can be appropriately added depending on the purpose, and as a result, it is possible to produce a coil spring having higher performance or more suitable for use. For example, the case where Cr is added will be described below.

Cr: 0.5-2.0%
Cr is effective in preventing decarburization, contributes to improvement in strength and resistance to temper softening, and is effective in improving fatigue resistance. It is also effective in improving warm sag resistance. For this reason, in this invention, it is preferable to contain 0.5 to 2.0% of Cr further. If the Cr content is less than 0.5%, these effects cannot be obtained sufficiently. On the other hand, if the Cr content exceeds 2.0%, the toughness is lowered, and cracks and surface scratches are likely to occur.

  FIG. 1 shows each manufacturing process. 1A to 1C are manufacturing steps for obtaining the compression coil spring of the present invention, and FIGS. 1D, 1E, and 1F are conventional examples. The manufacturing process shown in FIGS. 1A and 1B is a hot forming method using the following coiling machine, and the manufacturing process shown in FIG. 1C is a cold forming method using any coiling machine. is there.

  FIG. 2 shows an outline of the molding part of the coiling machine used in the manufacturing process shown in FIGS. 1 (A), (B) and (F). As shown in FIG. 2, the coiling machine forming unit 1 includes a feed roller 10 for continuously supplying the steel wire M, a coiling unit 20 for forming the steel wire M into a coil shape, and a rear after coiling a predetermined number of turns. A high frequency for heating the steel wire M between the cutting means 30 having the cutting blade 30a and the inner mold 30b for separating the steel wire M supplied continuously and the coiling tool 22 from the outlet of the feed roller 10. And a heating coil 40. The coiling unit 20 is for guiding the steel wire M supplied by the feed roller 10 to an appropriate position, and for processing the steel wire M supplied via the wire guide 21 into a coil shape. A coiling tool 22 including a coiling pin (or coiling roller) 22a and a pitch tool 23 for adding a pitch are provided.

  Rapid heating in the coiling machine is performed by the high-frequency heating coil 40, and the steel wire is heated to the austenite region within 2.5 seconds. The installation position of the high frequency heating coil 40 is as shown in FIG. The high-frequency heating coil 40 is installed in the vicinity of the wire guide 21, and the coiling portion 20 is provided so that the steel wire M can be formed immediately after heating. In addition, since the installation position of a high frequency heating coil should just be able to shape | mold immediately after heating the steel wire M, it may be other than the position shown in this embodiment.

  In the coiling portion 20, the steel wire M that has passed through the wire guide 21 is brought into contact with the coiling pin 22a and bent at a predetermined curvature, and further brought into contact with the downstream coiling pin 22a and bent at a predetermined curvature. Then, the steel wire M is brought into contact with the pitch tool 23 to give a pitch so as to obtain a desired coil shape. When the desired number of turns is reached, the cutting blade 30a of the cutting means 30 is cut by shearing with the straight portion of the inner mold 30b to separate the steel wire M supplied from the rear and the spring-shaped steel wire M from each other. .

(1) 1st Embodiment The manufacturing process of 1st Embodiment is shown to FIG. 1 (A). First, equivalent to a circle consisting of 0.45 to 0.80% C, 0.15 to 2.50% Si, 0.3 to 1.0% Mn, with the balance being iron and inevitable impurities. A steel wire M having a diameter of 1.5 to 10 mm is prepared. The steel wire M is supplied to the feed roller 10 by a wire drawing machine (not shown), and the steel wire M is heated to the austenite region within 2.5 seconds by the high-frequency heating coil 40, and then coiling is performed in the coiling section 20 (coiling). Process).

  At this time, during the period from heating to quenching, a hydrocarbon-based gas is directly sprayed onto the surface of the steel wire M to perform carburizing treatment simultaneously (carburizing step). For example, a gas spray nozzle 50 as shown in FIG. 3 is used. The gas spray nozzle 50 shown in FIG. 3 is located downstream of the high-frequency heating coil 40 and in the outer diameter direction of a coil spring formed from the steel wire M. In this case, the tip position of the gas spray nozzle 50 is preferably 0.1 to 10.0 mm from the surface of the steel wire M. If the tip position is less than 0.1 mm, the gas flow becomes worse due to pressure loss and a sufficient gas flow rate cannot be obtained, and the amount of gas that dissipates against the surface of the steel wire M is increased. As a result, the inner diameter of the steel wire M is increased. Since the amount of gas that goes around in the direction and contacts the surface of the steel wire M is reduced, carburization is not stable. Moreover, if the distance from the surface of the steel wire M is too short, the gas blowing nozzle 50 is deteriorated by the radiant heat from the steel wire M. On the other hand, if the tip position of the gas spray nozzle 50 exceeds 10.0 mm, it is necessary to spray a large amount of hydrocarbon gas, which is not economical. Carburization is performed at a gas spray pressure (dynamic pressure on the surface of the steel wire M) of 0.1 to 5.0 kPa and a steel wire temperature of 850 to 1150 ° C., and the maximum C concentration on the surface of the steel wire M is 0.7 to 0.00. A C concentrated layer having a thickness of 9% by weight and a thickness of 0.01 to 0.1 mm is formed. Thereby, the surface layer part 50HV or more higher than wire internal hardness can be obtained.

  Next, the coil that has been separated after coiling and is still in the austenite region is quenched as it is in a quenching tank (not shown) (for example, an oil of about 60 ° C. as a quenching solvent) (quenching step), and further tempered (for example, 150-450 ° C.) (tempering step). By performing quenching, a high-hardness structure composed of a martensite structure is obtained, and by further tempering, a tempered martensite structure having excellent toughness can be obtained. Here, a general method may be used for quenching / tempering, and the heating temperature of the wire before quenching, the type and temperature of the quenching solvent, and the temperature and time of tempering are appropriately set according to the material of the steel wire M. .

  Furthermore, desired fatigue resistance can be obtained by subjecting the steel wire M to a shot peening process (shot peening process) and a setting process (setting process). Since coiling is performed in a state heated to the austenite region, it is possible to prevent the generation of residual stress due to processing. For this reason, it is easy to apply compressive residual stress by shot peening, and deep and large compressive residual stress can be effectively applied on the inner diameter side of the spring. Furthermore, by performing the setting process, a deep compressive residual stress distribution is formed in the maximum principal stress direction when used as a spring, and fatigue resistance can be improved.

  In the present embodiment, a first shot peening process using a shot having a particle diameter of 0.6 to 1.2 mm, a second shot peening process using a shot having a particle diameter of 0.2 to 0.8 mm, A multi-stage shot peening process including a third shot peening process with a shot of 02 to 0.30 mm is performed. In the shot peening process to be performed later, since the shot smaller than the shot peening process to be performed earlier is used, the surface roughness of the wire can be smoothed.

  For shots used in shot peening, steel cut wires, steel beads, high hardness particles such as FeCrB, and the like can be used. Further, the compressive residual stress can be adjusted by a shot equivalent sphere diameter, a projection speed, a projection time, and a multi-stage projection method.

  Moreover, in this embodiment, hot setting is performed as a setting process, the shear strain amount acting on the surface of the wire rod is heated to 100 to 300 ° C., and the shear strain amount at the working stress when actually used as a spring is greater than the shear strain amount. Thus, plastic strain is applied to the spring-shaped steel material.

  The compression coil spring of the present invention produced by the process as described above has an internal hardness of 570 to 700 HV in an arbitrary cross section of the spring element wire, and has a C concentrated layer on the surface layer portion on the inner side of the coil spring. In the C-concentrated layer, the surface C concentration and the C-concentrated layer depth continuously decrease from the spring inner diameter side toward the outer diameter side along the cross-sectional circumference. The C-concentrated layer has a maximum C concentration of 0.7 to 0.9% by weight, a thickness of 0.01 to 0.1 mm, and a hardness that is 50 HV higher than the internal hardness. Therefore, the compression coil spring of the present invention is excellent in fatigue resistance because the compressive residual stress is deep and large.

  FIG. 5 is a view showing a modification of FIG. 3, in which a gas spray nozzle 50 is installed between the coils of the high-frequency heating coil 40. In this example, since the carburizing step is performed during heating before coiling, the carburizing time can be freely set by appropriately selecting the number of gas spray nozzles 50.

(2) Second Embodiment In the first embodiment, the carburizing process was performed during hot coiling. However, as shown in FIG. 1B, the compression coil of the present invention can be used even if the carburizing process is performed before hot coiling. A spring can be obtained. For example, as shown in FIG. 4, a gas blowing nozzle 50 is installed in front of the feed roller 10, and a high frequency heating coil 40 is arranged in front of it to perform the carburizing process. The gas blowing nozzle 50 is positioned upstream of the feed roller 10 and in the direction of the outer diameter side when formed into a spring shape in the radial direction of the steel wire M. The carburizing conditions are the same as in the first embodiment. After the carburizing process, the steel wire M is not cut off and used as it is in the coiling process. The coiling process, quenching process, tempering process, shot peening process, and setting process are performed in the same manner as in the first embodiment.

  According to the second embodiment, a compression coil spring equivalent to that of the first embodiment can be obtained. In the second embodiment, since the carburizing process is performed before coiling, the carburizing time can be freely set as compared with the first embodiment.

(3) Third Embodiment In addition, the compression coil spring of the present invention can be obtained using a cold forming method as shown in FIG. Cold coiling is performed on the steel wire M used in the first embodiment by an arbitrary coiling machine (coiling process). Then, the steel wire M after coiling is heated to the austenite region within 20 seconds and quenched (carburizing and quenching step). At this time, high-frequency heating means is used for heating, and a carburizing process is simultaneously performed by spraying a hydrocarbon-based gas directly on the surface of the steel wire M during heating and quenching. For example, as shown in FIG. 6, the steel wire M is fixed to a jig 60 that can rotate and move in the vertical direction, and the high-frequency heating coil 40 and the high-frequency heating coil 40 are adjacent to each other around the steel wire M. Gas spray nozzles 50 are respectively installed in Then, the steel wire M is rotated upward (or downward) by rotating the jig 60 and gas is supplied through the gas blowing nozzle 50 so that the surface of the coil spring is uniformly quenched and carburized. To do. The carburizing conditions are the same as in the first embodiment.

  Next, similarly to the first embodiment, a quenching process, a tempering process, a shot peening process, and a setting process are sequentially performed. Since heating is performed up to the austenite region in the heating and quenching step, the tensile residual stress generated by cold forming can be eliminated, and the effects of shot peening and setting can be obtained effectively. Thus, a compression coil spring having the same performance as that of the first embodiment can be obtained.

  Compared with the first and second embodiments, the third embodiment performs high-frequency heating on the coil-shaped steel wire M, so it is necessary to consider soaking. In addition, since the heating time is relatively long, the effect of crystal grain refinement is inferior to that of the first and second embodiments. In the cold forming method, a large processing strain remains in the coil spring after forming, and the processing strain is not uniform within the individual. For this reason, in the heating and quenching process, when the processing strain is released, the shape tends to be distorted. Furthermore, in the third embodiment, when heating a coil spring having a complicated shape (a deformed spring such as a conical shape, a bell shape, a double-end choke shape, a drum shape, a barrel shape, etc.) Heating coil is required, and a great deal of labor is required for designing the heating coil and determining heating conditions. Further, it may be difficult to equalize the temperature of the coil spring having a more complicated shape. Therefore, from any viewpoint, the manufacturing method in the first and second embodiments is more preferable than that in the third embodiment.

1. Sample Preparation Method Samples of coil springs were prepared by each manufacturing process, and fatigue resistance was evaluated. First, having a chemical composition shown in Table 1, the balance being prepared hard drawn wire and oil-tempered over line consisting of iron and inevitable impurities. The wire diameter of each wire is as shown in Table 2. The hard against drawn wire or oil-tempered over line, FIG. 1 (A) ~ (F) in the production process (respectively, represent the manufacturing process A-E) shown in accordance with, by hot molding or cold molding A coil spring having a spring index of 6, an effective portion pitch angle of 9 °, and an effective portion winding number of 4.25 was produced.

  In the manufacturing process A, coiling is performed by heating a steel wire with a coiling machine (see FIG. 3) equipped with a high-frequency heating coil and a gas spray nozzle, and after carburizing under the conditions shown in Table 2, oil at 60 ° C. Quenched by. In Table 2, the carburizing temperature is the surface temperature of the steel wire, and the dynamic pressure represents the dynamic pressure of propane gas on the surface of the steel wire. Then, the tempering process was performed on the conditions of Table 2 (Invention Examples 1-18, Comparative Examples 1-3). Moreover, in the manufacturing process B, after performing carburizing treatment using the coiling machine shown in FIG. 4 under the carburizing treatment conditions shown in Table 2, the steel wire is heated to 900 ° C. and coiled, and quenched with oil at 60 ° C. did. Thereafter, a tempering treatment was performed at 350 ° C. (Invention Example 19).

  In the manufacturing process C, after cold coiling with an arbitrary coiling machine, using a device as shown in FIG. 5, heat carburizing treatment is performed under the conditions described in Table 2, quenching with oil at 60 ° C., 350 A tempering treatment was performed at ° C (Invention Example 20). For comparison, a sample of a coil spring was manufactured by manufacturing steps D and E. In the manufacturing process D, after cold coiling, the annealing process was performed at the temperature shown in Table 2 (Comparative Examples 4-6). In manufacturing step E, after cold coiling, annealing was performed at 400 ° C., and then nitriding was performed. In the nitriding treatment, a hard layer having a depth of 0.04 mm was formed on the surface of the wire. In the manufacturing process F, the steel wire was hot-formed without introducing propane gas in the apparatus used in the manufacturing process A, quenched with oil at 60 ° C., and then tempered at 350 ° C. Next, the coil spring was housed in a batch-type heating furnace and carburized (Comparative Example 7).

  Next, a shot peening process and a setting process were performed on each sample. In the shot peening process, a first shot peening process using a steel round cut wire having a sphere equivalent diameter of 1.0 mm, a second shot peening process using a steel round cut wire having a sphere equivalent diameter of 0.5 mm, and a sphere equivalent diameter. A third shot peening treatment with 0.1 mm steel beads was sequentially performed. The setting was hot setting, and the heating was performed at a coil spring heating temperature of 200 ° C. and a load stress of 1500 MPa.

2. Evaluation Method Various properties of the sample thus obtained were investigated as follows. The results are shown in Table 3.

(1) Hardness (HV)
Using a Vickers hardness tester (Futuretec FM-600), the measurement was performed on the coil inner diameter side in the cross section of the wire rod of the coil spring. The measurement load was 10 gf from the surface to a depth of 0.05 mm, 25 gf from a depth of 0.05 to 0.1 mm, and 200 gf at a depth of 0.2 mm or more.

(2) Compressive residual stress ( _{−σ R0.2} , _{−σ R0.4} ), depth _0.2 , _{0.4 mm} , maximum compressive residual stress (−σ _Rmax ), integrated compressive residual stress (I _−σR ) Crossing point (CP)
On the inner diameter side surface of the coil spring, an X-ray diffraction type residual stress measuring device (manufactured by Rigaku) is used to measure the compressive residual stress in the + 45 ° direction (substantially the maximum principal stress direction when a compressive load is applied to the spring). ). The measurement was performed with a tube: Cr and a collimator diameter: 0.5 mm. In addition, the above measurement is performed after the entire surface of the wire is chemically polished using hydrochloric acid with respect to the coil spring, and the residual stress distribution in the depth direction is obtained by repeating this, and 0.2 mm, 0.4 mm from the surface is obtained from the result. The compressive residual stress at no load, the maximum compressive residual stress, and the crossing point at a depth of 5 mm were obtained. The integrated compressive residual stress value was calculated by integrating the compressive residual stress from the surface to the crossing point in the relationship diagram of depth and residual stress.

(3) Surface C concentration (Cc), C concentrated layer thickness (Ct)
The surface C concentration and the thickness of the C enriched layer were measured on the inner diameter side in the wire rod cross section of the coil spring. EPMA (Shimadzu Corporation EPMA-1600) was used for the measurement, and a line analysis was performed with a beam diameter of 1 μm and a measurement pitch of 1 μm. The C concentrated layer thickness was the depth from the surface until the same C concentration as the inside of the wire was reached. As for Invention Examples 8 and 15, as shown in FIG. 7 (A), the coil spring cross section from the position in the inner diameter direction (0 °) to the outer diameter direction (180 °) along the circumference of the cross section. The surface C concentration of each part was measured. 7B shows Invention Example 8 and FIG. 7C shows Invention Example 15, and the portion sandwiched between the two curves shows the C-enriched layer.

(4) Old austenite grain average grain size number (G)
As a pretreatment, a coil spring sample was heated at 500 ° C. for 1 hour. Then, at the position of the depth d / 4 of the cross section of the coil spring, the number of fields of view is 10 and the optical microscope (NiKON ME600) is used and the magnification is 1000 times and the measurement is performed according to JIS G0551. The grain average grain size number G was calculated.

(5) Surface roughness (Rz (maximum height))
Surface roughness was measured according to JIS B0601 using a non-contact three-dimensional shape measuring apparatus (MITAKA NH-3). The measurement conditions were: measurement magnification: 100 times, measurement distance: 4 mm, measurement pitch: 0.002 mm, cut-off value: 0.8 mm.

(6) Average crystal grain size (d _GS )
The average crystal grain size was measured by JEOL JSM-7000F (TSL Solutions OIM-Analysis Ver. 4.6) by the FE-SEM / EBSD (Electron Back Scatter Diffraction) method. Here, the measurement was performed at the position of the depth d / 4 of the cross section of the coil spring, the observation magnification was 10,000 times, and the average crystal grain size was calculated with the boundary having an azimuth angle difference of 5 ° or more as the grain boundary.

(7) Fatigue resistance (breakage rate)
A fatigue test was performed at room temperature (in the air) using a hydraulic servo type fatigue tester (Enomiya Seisakusho). The test stress was 735 ± 662 MPa, the frequency was 20 Hz, the number of tests was 8 each, and the fatigue resistance was evaluated by the breakage rate (number of breaks / number of tests) at the time of vibration 20 million times.

3. Evaluation Results (1) Hardness As can be seen from Table 3, high fatigue resistance is obtained when the internal hardness is 570 to 700 HV (more preferably 570 HV to 690 HV) in the present invention by the hot forming method. On the other hand, from the results of Comparative Example 3 (high tempering temperature), even a coil spring manufactured by hot forming and carburized on the inner diameter side has sufficient fatigue resistance when the hardness is less than 570 HV. (Reason: poor strength against fatigue resistance required in this technical field). Moreover, in all the invention examples, the hardness of the inner diameter side surface is increased by 50 HV or more compared to the inside due to carburization. Thereby, a high compressive residual stress can be obtained in the vicinity of the surface, and the occurrence of fatigue cracks starting from the vicinity of the surface (including the outermost surface) can be prevented (improvement in fatigue resistance).

(2) Old austenite grain average crystal grain size In Invention Examples 1 to 4 by production method A consisting of materials A, B, C, or D having a simple composition, G is No. 10 or more, and V amount has an effect of grain refinement As a result, fine crystal grains comparable to those of Comparative Examples 4 and 5 made of high-grade steel are obtained. Therefore, it is estimated that the fatigue resistance is improved in Invention Examples 1 to 4. The reason why such fine crystal grains are obtained using a material having a simple composition is due to rapid heating by high-frequency heating. That is, heating in a short time by high-frequency heating leads to suppression of coarsening or refinement of prior austenite grains, and in Invention Examples 1 to 4 having a simple composition, G obtains fine crystal grains having a number of 10 or more. It has good fatigue resistance.

  In the comparative example 7 by the manufacturing method F which consists of the material C of a simple composition, since the furnace carburizing is performed after the hot forming method, compared with the manufacturing method AC which is performing the carburizing process in a short time, the old austenite The grain average grain size is significantly reduced. Even with a spring made by hot forming and carburized over the entire surface, if the prior austenite grain average grain size is less than 10, sufficient fatigue resistance cannot be obtained.

  Also in Invention Example 20 by production method C, as a result of short-time heating by high-frequency heating, G was 10.1 and fine crystal grains could be obtained. Compared with the manufacturing method A, the grain size of the manufacturing method C is slightly deteriorated because the manufacturing method C performs high-frequency heating on the coil-shaped material, so that the steel wire rod as in the manufacturing method A is heated. This is because, when soaking is considered, the heating time becomes long although it is not as high as the manufacturing method F. In other words, production method A is more preferable than production method C in terms of crystal grain refinement.

(3) Average crystal grain size In invention examples 1 to 4 consisting of materials A, B, C, or D having a simple composition, d _GS is 0.66 to 0.89 μm, and Comparative Example 5 using high-grade steel The average crystal grain size was about the same as 6. The reason for this is that, as described above, heating in a short time by high-frequency heating led to suppression of coarsening of the structure or miniaturization, and as a result, in Examples 1 to 4, the fine average crystal grain size Is obtained and fatigue resistance is improved.

In Comparative Example 7 by the manufacturing method F, since the furnace carburizing is performed after the hot forming method, the average crystal grain size is significantly larger than the manufacturing methods A to C in which the carburizing treatment is performed in a short time. Even with a spring made by a hot forming method and carburized over the entire surface, sufficient fatigue resistance cannot be obtained if the average crystal grain size (d _GS ) exceeds 2.0 μm.

Also in Invention Example 20 by the production method C, as a result of short-time heating by high-frequency heating, d _GS is 0.94 μm and fine crystal grains can be obtained. As described above, in the manufacturing method C, the heating takes a long time compared to the manufacturing method A. Therefore, the manufacturing method A is more preferable than the manufacturing method C in terms of crystal grain refinement.

(4) Surface C concentration, C concentrated layer thickness In Invention Examples 1 to 20, the surface C concentration is 0.7 to 0.9% on the spring inner diameter side, and the C concentrated layer thickness (the same as the inside of the wire) Carburization with a depth of 30 μm or more is carried out and the hardness near the surface is high, so that a high compressive residual stress is obtained near the surface. Moreover, high fatigue resistance can be obtained by improving the surface roughness.

  Table 4 shows the surface C concentration with respect to the C concentration measurement position in Invention Example 8 and Invention Example 15, and FIG. 8 is a graph of Table 4. As shown in FIG. 8, the surface C concentration continuously decreases from the inner diameter direction of the coil spring to the outer diameter direction along the circumference of the cross section. Further, as shown in FIGS. 7B and 7C, the C concentrated layer depth continuously decreases from the inner diameter side toward the spring outer diameter side along the cross-sectional circumference. Thus, since the surface C concentration is higher and the C enriched layer depth is deeper on the inner diameter direction side of the cross section of the coil spring, the yield stress is higher. Therefore, since greater compressive residual stress is applied by shot pinning, the fatigue strength is enhanced.

(5) Residual stress distribution In Invention Example 3 produced by Manufacturing Method A, Invention Example 19 prepared by Manufacturing Method B, and Invention Example 20 prepared by Manufacturing Method C using the same wire material, annealing treatment was performed so as to obtain equivalent hardness. Compared with Comparative Example 4 performed, the compressive residual stress ( _{−σ R0.4} ) at a deep position from the surface is large. The reason is that in the invention example produced by the manufacturing method A or B, the tensile residual stress (remaining on the coil inner diameter side) generated in the cold coiling hardly occurs in the hot coiling, and the manufacturing method C is used. In the inventive example 20, the tensile residual stress generated in the cold coiling is completely eliminated by heating to the austenite region thereafter. That is, compared with Comparative Example 4 in which a tensile residual stress was generated by cold coiling, in Invention Examples 3, 19 and 20, the compressive residual stress due to shot peening easily enters deep from the surface, and tends to be a fracture starting point. Since the compressive residual stress at a depth of 4 mm can be increased, fatigue resistance can be improved.

For Invention Examples 1 to 20, all of -σ _Rmax is ₉₀₀ MPa or more, and the yield stress in the vicinity of the surface is increased by carburization, so that compressive residual stress is greatly applied by _{shot peening} and I- _σR is 150 MPa. -Mm or more, CP is 0.45 mm or more, and a deep and large compressive residual stress is obtained. Therefore, it can be seen that the fatigue resistance is improved in the inventive examples.

  In Comparative Example 7 in which a general furnace carburizing process was performed after hot coiling, the application of residual stress due to shot pinning is biased near the surface due to the large thickness of the carburized layer. As a result, the compressive residual stress at a depth of 0.1 to 0.4 mm, which tends to be a fracture starting point, is reduced.

As a result of observing the fracture surface of the broken products of Comparative Examples 2 to 4, the fracture starting point was within a range of 0.15 to 0.35 mm in depth from the surface, and was an internal starting point starting from a nonmetallic inclusion. It was. This depth corresponds to the vicinity of the region where the maximum value of the combined stress (working stress-residual stress) appears, and the compressive residual stress in the region (-σR _0.2 , -σR _0.4 as an index) is large. Is important for fatigue resistance. For this reason, in Invention Examples 1 to 20 in which -σR _0.2 is 200 MPa or more and -σR _0.4 is 60 MPa or more, a comparative example in which an expensive wire to which a higher element is added is used and nitriding treatment is performed. High fatigue resistance of 6 or more can be obtained.

(6) Surface Roughness Inventive Examples 1 to 20 in which high fatigue resistance is obtained, the surface roughness Rz (maximum height) is 9.0 μm or less, and sufficiently satisfies the desired surface roughness Rz of 20 μm or less. ing. Here, when Rz exceeds 20 μm, a valley portion in the surface roughness becomes a stress concentration portion, and cracks are generated and propagated from the valley portion as a starting point, resulting in premature breakage. The surface roughness is formed by rubbing with tools during coiling or by shot peening. The size of the surface roughness formed by the shot peening process is determined by a combination of the hardness of the wire and conditions such as the particle size and hardness of the shot and the projection speed. Therefore, it is necessary to appropriately set shot peening conditions in which Rz does not exceed 20 μm. In Invention Examples 3 and 7 to 15, the surface roughness is small as compared with Comparative Example 3 having the same internal hardness. This is because a C-concentrated layer having high hardness is formed on the surface. Therefore, the improvement of the surface hardness by forming the carburized layer is effective for the suppression of the surface roughness that tends to become the starting point of fracture, that is, the improvement of the reliability by improving the fatigue resistance.

(7) Carburizing conditions It is confirmed that the gas blowing pressure (dynamic pressure on the surface of the wire) is preferably 0.5 kPa to 5.0 kPa, and the steel wire temperature during gas blowing is preferably 850 to 1150 ° C. It was. According to these conditions, as shown in Invention Examples 7 to 15, the surface C concentration is 0.7% or more, and a C concentrated layer thickness of 0.01 mm or more is obtained. From Table 2, a gas dynamic pressure of 5.0 kPa or less is sufficient. Therefore, if the gas dynamic pressure exceeds 5.0 kPa, it is not economical. In addition, if the gas dynamic pressure increases, the temperature drop of the steel wire rod due to gas blowing increases, and the amount of heat input required is accordingly increased. To increase.

  From the viewpoint of the rate of carburization reaction, carburizing in a short time requires a steel wire temperature of 850 ° C. or higher. In Comparative Example 1 where the steel wire temperature is 800 ° C., no C-enriched layer is obtained. On the other hand, in Comparative Example 4 where the steel wire temperature is high, although the carburization reaction has occurred sufficiently, the grain size deteriorates due to the high heating temperature, and the fatigue resistance is reduced.

(8) Dimensional accuracy The invention example by manufacturing method A and manufacturing method B is preferable for components that require strict dimensional accuracy. In manufacturing method C, although durability is good, dimensional accuracy is inferior to manufacturing method A and manufacturing method B. The reason for this is that large deformation strain remains in the coil spring after cold forming, and the deformation strain is not uniform within the individual, so that the strain is released by heating to the austenite region afterwards. This is because inconveniences such as non-uniform deformation and large distortion of the shape occur. On the other hand, in the manufacturing method A and the manufacturing method B, processing distortion does not remain by hot forming.

  The difference in dimensional accuracy between production method A and production method B and production method C was evaluated using 50 coil springs after quenching. As a result, the coil diameter was 0.047 mm for the coil produced by production methods A and B, whereas the standard deviation of the coil produced by production method C was 0.047 mm.

  The present invention is a carbon steel wire, hard steel wire, piano wire, spring steel wire, carbon steel oil temper wire, chrome vanadium steel oil temper wire, silicon chrome steel oil temper wire, silicon chrome vanadium steel oil temper used as a spring. Applicable to lines. In particular, it is preferable to apply to inexpensive carbon steel wires, hard steel wires, piano wires, and spring steel wires. This is due to hot coiling, so there is no need to use the expensive oil tempered wire used in conventional cold forming, and the conventional cold forming spring using high-grade steel with an inexpensive wire rod This is because fatigue resistance exceeding 1 is obtained.

DESCRIPTION OF SYMBOLS 1 ... Coiling machine shaping | molding part, 10 ... Feed roller, 20 ... Coiling part, 21 ... Wire guide, 22 ... Coiling tool, 22a ... Coiling pin, 23 ... Pitch tool, 30 ... Cutting means, 30a ... Cutting blade, 30b ... Inside Mold, 40 ... high frequency heating coil, 50 ... nozzle, 60 ... jig, M ... steel wire.

Claims (19)

  In a compression coil spring using a steel wire having a mass% and C of 0.80 wt% or less, the spring inner diameter side surface layer portion has a C-concentrated layer exceeding the average concentration of C contained in the steel wire, A compression coil spring characterized in that the surface C concentration and the C-enriched layer depth continuously decrease from the spring inner diameter side toward the outer diameter side along the circumference of the wire rod in the cross section of the wire.   2. The compression coil spring according to claim 1, wherein an internal hardness in an arbitrary wire cross section is 570 to 700 HV, and a hardness of the C concentrated layer on a spring inner diameter side is 50 HV or more higher than the internal hardness. .   The maximum C concentration in the C concentrated layer on the inner diameter side of the spring is 0.7 to 0.9% by mass, and the thickness of the C concentrated layer is 0.01 to 0.1 mm. The compression coil spring according to claim 1 or 2. The compression coil spring has prior austenite crystal grains, and an average crystal grain size number of the prior austenite crystal grains defined in JIS G0551 is 10 or more. Compression coil spring. The compression coil spring has crystal grains having an azimuth angle difference of 5 ° or more, and an average crystal grain size of the crystal grains having the azimuth angle difference of 5 ° or more measured by SEM / EBSD method is 2.0 μm or less. The compression coil spring according to any one of claims 1 to 4.   6. The maximum compressive residual stress at no load is 900 MPa or more in a substantially maximum principal stress direction generated when a compression load is applied to a spring on the inner diameter side of the coil spring of the wire rod. A compression coil spring according to claim 1.   The compressive residual stress at a depth of 0.2 mm from the surface of the wire under no load is 200 MPa or more in the direction of the substantially maximum principal stress generated when a compression load is applied to the coil spring inner diameter side spring of the wire. The compression coil spring according to any one of claims 1 to 6, wherein a compressive residual stress at a depth of 0.4 mm from the surface is 60 MPa or more. The crossing point is the depth from the surface of the wire where the value of the compressive residual stress at no load is zero in the direction of the substantially maximum principal stress generated when compressive stress is applied to the coil spring inner diameter side spring of the wire. When the integrated value from the surface to the crossing point is expressed as I _−σR in the residual stress distribution curve with the residual stress on the vertical axis and the wire radius on the horizontal axis, I _−σR is 150 MPa · mm or more. The compression coil spring according to any one of claims 1 to 7.   The compression coil spring according to any one of claims 6 to 8, wherein the compressive residual stress is applied by shot peening. The shot peening treatment includes a first shot peening treatment with a particle size of 0.6 mm to 1.2 mm, a second shot peening treatment with a shot with a particle size of 0.2 mm to 0.8 mm, and a particle size of 0.02 mm. 10. The compression coil spring according to claim 9 , wherein the compression coil spring is a multi-stage shot peening process including a third shot peening process using a shot of 0.30 mm to 0.30 mm. Spring shape, cylindrical, or conical, across the diaphragm type, the compression coil spring according to claim 1, wherein the bell-shaped, hourglass-shaped, is either barrel. A coiling process in which a steel wire is hot-formed by a coil spring forming machine, a quenching process in which a coil that is cut after coiling and still in the austenite region is quenched, a tempering process in which the coil is tempered, and a wire surface In the manufacturing method of the compression coil spring in which the shot peening process for imparting compressive residual stress and the setting process are sequentially performed,
The coil spring forming machine is continuously supplied from the rear after a feed roller for continuously supplying a steel wire, a coiling portion for forming the steel wire into a coil shape, and coiling the steel wire with a predetermined number of turns. A steel wire and cutting means for cutting,
The coiling unit is for guiding a steel wire supplied by the feed roller to an appropriate position of a processing unit, and for processing the steel wire supplied via the wire guide into a coil shape. A coiling tool consisting of a coiling pin or a coiling roller and a pitch tool for adding a pitch are provided.
The coil spring forming machine further has heating means for raising the temperature of the steel wire to the austenite region within 2.5 seconds between the coiling tool from the feed roller outlet,
Between heating and quenching, hydrocarbon gas is blown directly onto the surface of the steel wire from the gas blowing nozzle located in the direction of the outer diameter when formed into a spring shape in the radial direction of the steel wire. A method of manufacturing a compression coil spring characterized by performing a carburizing step.   The heating means is high-frequency heating, and is concentric with the steel wire on the passage of the steel wire in the wire guide or on the passage of the steel wire in the space between the steel wire outlet end and the coiling tool in the wire guide. The method of manufacturing a compression coil spring according to claim 12, wherein a high-frequency heating coil is arranged so as to be. A carburizing process for forming a C-enriched layer on the surface layer of the steel wire, a coiling process for hot forming the steel wire with a coil spring forming machine, and a coil that has been separated after coiling and is still in the austenitic region is quenched as it is. In the manufacturing method of the compression coil spring that sequentially performs the quenching process, the tempering process to condition the coil, the shot peening process to impart compressive residual stress to the wire surface, and the setting process,
The means for forming the C-enriched layer in the carburizing step is carbonized on the surface of the steel wire heated from the gas spray nozzle located in the direction of the outer diameter when formed into a spring shape in the radial direction of the steel wire. Hydrogen gas is directly blown,
The coil spring forming machine used in the coiling process is continuously connected from the rear after a feed roller for continuously supplying a steel wire, a coiling portion for forming the steel wire into a coil shape, and coiling the steel wire with a predetermined number of turns. A steel wire that is supplied and a cutting means for cutting,
The coiling unit is for guiding a steel wire supplied by the feed roller to an appropriate position of a processing unit, and for processing the steel wire supplied via the wire guide into a coil shape. A coiling tool composed of a coiling pin or a coiling roller and a pitch tool for adding a pitch are provided.
The coil spring forming machine further has heating means for raising the temperature of the steel wire to the austenite region within 2.5 seconds between the coiling tool from the feed roller outlet,
The heating means is high-frequency heating, and is concentric with the steel wire on the passage of the steel wire in the wire guide or on the passage of the steel wire in the space between the steel wire outlet end and the coiling tool in the wire guide. A high frequency heating coil is arranged so that
A method of manufacturing a compression coil spring, wherein the carburizing step and the coiling step are continuous steps in which there is no separation of the steel wire in the middle. A coiling process for forming a steel wire with a coil spring forming machine, a heating and quenching process in which the coil is heated to an austenite region within 20 seconds and quenched, a tempering process in which the coil is tempered, and a compressive residual stress on the surface of the wire In the manufacturing method of the compression coil spring in which the shot peening process to be applied and the setting process are sequentially performed,
The heating means in the heating and quenching step is high frequency heating,
A compression coil spring comprising a carburizing process in which a hydrocarbon-based gas is directly blown onto a steel wire surface from a gas blowing nozzle positioned in an outer diameter direction of a formed coil spring between heating and quenching. Production method.   The steel wire material surface temperature at the time of blowing the hydrocarbon gas is 850 to 1150 ° C., and the dynamic pressure of the hydrocarbon gas on the steel wire surface portion is 0.1 to 5.0 kPa. A method for manufacturing a compression coil spring according to any one of claims 12 to 15.   12. The compression coil spring according to claim 1, wherein the surface C concentration of the spring inner diameter side surface layer portion is 0.05% or more higher than the surface C concentration of the spring outer diameter side surface layer portion.   18. The thickness of the C concentrated layer on the inner diameter side of the spring is 4 μm or more thicker than the thickness of the C concentrated layer on the outer diameter side of the spring. 18. Compression coil spring.   16. A carburizing step is performed, in which a surface of a steel wire rod on the outer diameter side sprayed with a hydrocarbon gas is cooled while a hydrocarbon gas is introduced into the surface of the steel wire rod on the inner diameter side to cause a reaction. A method for manufacturing a compression coil spring.

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